1. Field of the Invention
The present invention relates to a retardation film (or retardation plate) using a novel liquid crystal.
2. Background Art
The STN-mode liquid crystal display device comprises an STN-mode liquid crystal cell, two polarizing plates (or polarizing films)) and one or two optical compensation sheet(s) (retardation film) provided between the STN-mode liquid crystal cell and the polarizing plate.
The liquid crystal cell comprises a rod-like liquid crystalline molecule, two substrates for enclosing the rod-like liquid crystalline molecule, and an electrode layer for applying a voltage to the rod-like liquid crystalline molecule. In the STN-mode liquid crystal cell, an alignment film for orienting (aligning) rod-like liquid crystalline molecules is provided on two substrates. Furthermore, the rod-like liquid crystalline molecules are oriented in twisted alignment at 180 to 360° by using a chiral agent. The STN-mode liquid crystal display device is characterized in that a large-capacity and sharp display can be obtained by time-division driving even with a simple matrix electrode structure not having an active device (e.g., thin-film transistor, diode).
In an STN-mode liquid crystal display device having no retardation film, the display image is colored to blue or yellow due to birefringence of the rod-like liquid crystal molecule. The coloration of the display image is disadvantageous to both the monochromatic display and the color display. The retardation film is used to eliminate such coloration and obtain a bright and sharp image. The retardation film is sometimes imparted also with a function of enlarging the viewing angle of the liquid crystal cell. For the retardation film, a stretched birefringent film has been conventionally employed. The retardation film for STN-mode liquid crystal display devices, obtained by using a stretched birefringent film, is described in Patent Document 1 (JP-A-7-104284 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and Patent Document 2 (JP-A-7-13021).
Patent Document 3 (JP-A-2001-166147) discloses a discotic liquid crystal having a large refractive index anisotropy, however, the wavelength dispersion property is worsened and the improvement of performance is insufficient. In general, the wavelength dispersion property and the refractive index anisotropy are in a trade-off relationship and when the refractive index anisotropy is increased, the wavelength dispersion property is deteriorated. Development of a technique for getting rid of this trade-off is being demanded.
Non-Patent Documents 1, 2 and 3 (Molecular Crystals and Liquid Crystals, Vol. 357, page 55 (2001); Organic Letters, Vol. 4, page 157 (2002); and Liquid Crystals, Vol. 29, page 899 (2002), respectively) disclose a bis-form discotic liquid crystal, but these documents are completely silent on the optical performance. Furthermore, this liquid crystal is colored to yellow due to its long conjugate system or contains a mercury and therefore, the practical use thereof as an optical member is improper. Also, Patent Document 4 (JP-A-8-327822) discloses a general formula for bis-form compounds of triphenylene, but in all of these compounds, the triphenylene ring has many aromatic rings (mainly a benzene ring) in the side chain and the molecule has a very large molecular weight, giving rise to problems such that the orientation proceeds slowly due to high viscosity and the refractive index anisotropy is small due to large molecular weight.